1. Field of the Invention
This invention relates to the field of droplet or particle cluster formation and in particular to a method of shaping and inducing a density change in a particle cluster by means of a combination of acoustic and optical energy.
2. Description of the Prior Art
The word xe2x80x9cparticlexe2x80x9d in this document shall denote a volume element that contains a single body of material that is in either a liquid or a solid phase. A particle has a mass density and a shape, which is defined by the surface boundary of the material. Two examples of particles are (1) a liquid droplet or (2) a metal crystal. The word xe2x80x9cparticle clusterxe2x80x9d in this document denotes a plurality of particles that are close enough one to another as to influence each other""s motion either directly (through collisions) or indirectly (due to interactions with external forces). A particle cluster has an average number density and a shape, which defines the spatial distribution of particles. Two examples of particle clusters are (1) a group of several liquid droplets and (2) a group of several metal crystals.
Acoustic levitation of an object within a chamber has heretofore been accomplished by the use of one or a few acoustic standing wave patterns, wherein the acoustic wavelength was between about one-quarter and twice the length of the chamber. The chamber had to have highly sound reflective walls to provide a Q (a measure of sound reflectance) of at least about fifty. The object remained at a region of low acoustic pressure, because as it drifted in a particular direction, radiation pressure of the standing wave pattern pushed the object back. While harmonics of a fundamental or lowest frequency could be used, these higher harmonics restricted the size of the object. For these acoustic levitators, the object size had to be small compared to the acoustic wavelength, such as no more than about 20% of the wavelength. U.S. Pat. No. 4,573,356 describes the general state of the art of the use of acoustic standing wave patterns to levitate objects and is incorporated by reference.
The prior art use of acoustic standing wave patterns, involved the use of one or only a few transducers which all emitted sound of relatively long wavelengths within a high Q chamber. A large sample requires a very long wavelength and long chamber. It is difficult to produce high intensity sound of long wavelengths and corresponding low frequencies. The force that could be applied to a levitated object was limited by the small number of transducers that could be easily used. Movement and shaping of the object required complicated control or required alteration of the chamber dimensions.
Acoustically levitating an object with acoustic energy from a large number of transducers to avoid the need for a chamber of high Q for simplified control of the position, shape of large objects with respect to the sound wavelength is known in the art. Guigne et al. U.S. Pat. No. 5,500,493 describes such an acoustic levitation apparatus. Acoustic energy is used to position an object, which simplifies the application of forces in defined directions to the object and which allows the application of large forces to the object. The system includes transducers that direct separate acoustic beams at the object with the system constructed so the beams do not create standing wave patterns. A plurality of beams whose phases at the object are not closely controlled, are directed at different surface areas of the object so the beams do not substantially overlap at the object and create possible canceling effects. A very large force is applied to the bottom of an object lying in a gravity environment, by directing a plurality of beams at the same area at the bottom of the object, and with the beams being controlled so they are substantially in phase at the object area. This plurality of beams can also replace one or all of the transducers to provide much stronger forces to position and manipulate the object. The wavelength of the acoustic energy in each beam is preferably much less than one-tenth the diameter of the object in order to obtain efficient momentum transfer of energy to the object. Guigne, however, fails to recognize that such a system can be used for controlling the shape or average number density of particle clusters.
Kaduchak et al U.S. Pat. No. 6,467,350 is also directed to acoustic levitation of particles. However, Kaduchak did not recognize the utility of acoustically shaping the particle cluster for the purpose of rapid prototyping. The standing-wave field produced by an acoustic levitation device is strongly dependent upon the spatial alignment of the system components and often requires moderate to high electrical input power levels to drive the acoustic generators and achieve the desired levitation. This is especially true for levitating solid and liquid samples in air. To achieve the foregoing Kaduchak employed a method for concentrating particles suspended in a fluid including the steps of matching the distance between reflector and radiating element or between two radiating elements, i.e. tuning the resonant levitation cavity, to the acoustic resonance of the interior volume thereof when filled with the fluid; applying periodic electrical excitation to the acoustic radiating element (i.e. a piezoelectric transducer) such that resonant acoustic waves are generated in the interior volume of the levitation cavity, and subjecting the fluid having particles suspended therein to the steady-state force pattern formed by the resonant acoustic waves such that the particles move to the region of the steady-state force pattern and are concentrated.
Kaduchak also disclosed an apparatus for concentrating particles suspended or entrained in a fluid comprising a cylindrical piezoelectric transducer having a hollow interior portion and wherein the breathing-mode acoustic resonance of the cylindrical piezoelectric transducer is matched to the acoustic resonance of the interior portion thereof when the interior portion or levitation cavity is filled with the fluid. A function generator applies periodic electrical excitation to the surface of the cylindrical piezoelectric transducer such that resonant acoustic waves in are generated in the hollow interior portion of the cylindrical piezoelectric transducer. A means is provided for introducing the fluid having particles suspended or entrained therein into the region of the equilibrium force pattern formed by the resonant acoustic waves such that the particles move to the region of the equilibrium force pattern and are concentrated.
Photopolymers are well known in the art and have been used for the construction of various devices. For example extensive use of photopolymers has been made in printing. In flexographic printing as one example, also known as relief printing, ink is transferred from a pool of ink to a substrate by way of a printing plate. The surface of the plate is shaped so that the image to be printed appears in relief, in the same way that rubber stamps are cut so as to have the printed image appear in relief on the surface of the rubber. Typically, the plate is mounted on a cylinder, and the cylinder rotates at high speed such that the raised surface of the printing plate contacts a pool of ink, is slightly wetted by the ink, then exits the ink pool and contacts a substrate web, thereby transferring ink from the raised surface of the plate to the substrate to form a printed substrate.
Photopolymerizable resin compositions generally comprise an elastomeric binder, herein sometimes referred to as a prepolymer or an oligomer, at least one monomer and a photoinitiator. To prepare the plates, there is generally formed a photopolymerizable layer interposed between a support and one or more cover sheets that may include slip and release films to protect the photosensitive surface. Prior to processing the plate, the cover sheets may be removed, and the photosensitive surface is exposed to actinic radiation in an imagewise fashion. Upon imagewise exposure to actinic radiation, polymerization, and hence, insolubilization of the photopolymerizable layer occurs in the exposed areas. Treatment with a suitable developer removes the unexposed areas of the photopolymerizable layer leaving a printing relief, which can be used for flexographic printing.
Many different elastomeric materials have been investigated for the preparation of the photopolymer plates. These include polyamide-based photopolymer (containing a polyamide as an essential component which dissolves or swells in a washout solution (typically, water, alkaline aqueous solution, or alcohol), a polyvinyl alcohol-based photopolymer (containing polyvinyl alcohol as an essential component), a polyester-based photopolymer (containing a low-molecular weight unsaturated polyester as an essential component), an acrylic-based photopolymer (containing a low-molecular weight acrylic polymer as an essential component), a butadiene copolymer-based photopolymer (containing a butadiene or isoprene/styrene copolymer as an essential component), or a polyurethane-based photopolymer (containing polyurethane as an essential component). Methacrylate- or acrylate-terminated polyurethane oligomers diluted with various acrylate or methacrylate monomers, along with a photoinitiator, are described in U.S. Pat. Nos. 4,006,024 and 3,960,572, which are incorporated herein by reference. The polyurethane oligomers of the ""024 and ""572 patents are derived from a diisocyanate such as toluene diisocyanate (TDI) and various polyester polyols or polyether polyols such as polypropylene glycol adipate, polyethylene oxide/propylene oxide copolymer, or a mixture thereof. U.S. Pat. Nos. 4,057,431, 4,139,436, 4,221,646 and 3,850,770, which are all incorporated by reference, all teach the use of photosensitive ethylenically unsaturated polyether urethanes for the production of printing plates. In each of these patents, the polyether is either polyethylene oxide, polypropylene oxide or a copolymer of the two. U.S. Pat. No. 5,228,571, which is incorporated herein by reference in its entirety, teaches the use of photosensitive ethylenically unsaturated polyether urethanes for the production of printing plates wherein at least 20% of the polyether diol is specifically poly(tetrahydrofuran) (polyTHF).
However, prior to the instant invention, the utility of combining an acoustic shaping and photopolymerization was not known. There is a need in the art for new methods of creating microfabricated components for electronics, optics and other applications.
It is an object of the invention to provide a process for achieving the control of the shape of a particle or particle cluster via acoustic levitation and acoustic manipulation. One use of this process is to expose the particle or particle cluster to electromagnetic radiation for the purposes of inducing a change in its molecular structure. One application of this process is to use UV radiation to crosslink a photopolymer droplet. This may be useful in the rapid prototyping industry as a new means of microcomponent fabrication.
Acoustic energy is used to control the shape of a particle or particle cluster for the purpose of inducing a phase and density change as a result of exposure to radiation. This process, known as optical acoustic molding, employs pairs of opposing ultrasonic transducers positioned around a particle or particle cluster to generate standing waves. These standing waves apply forces to points on the particle""s surface. The locus of such points represents a three-dimensional pressure function, which will ultimately cause the particle or particle cluster to acquire a predefined shape. Once the particle or particle cluster has attained the desired shape or density, a radiation source induces rapid melting or solidification (i.e. rapid change in density) of the particles.
More particularly, the illustrated embodiment of the apparatus positions a particle or particle cluster within a contained volume. The apparatus comprises an acoustic transducer means or system for generating acoustic energy to alter the shape of the particle or particle cluster. The acoustic transducer means comprises a single or triple axis acoustic levitation system. A controller coupled to the acoustic transducer means controls the acoustic transducer means to generate standing waves in the contained volume to levitate the particle or particle cluster and to shape the particle or particle cluster. A radiation source delivers radiation to the particle or particle cluster to induce a change in density in the particle or particle cluster. In this specification xe2x80x9cdensityxe2x80x9d is defined as xe2x80x9cmass densityxe2x80x9d in the case of the particle, and as xe2x80x9caverage number densityxe2x80x9d in the case of the particle cloud or plurality of particles.
In the illustrated embodiment the particle or particle cluster contains one or more droplets formed from a photopolymerizable solution. The suspension means is an acoustic suspension means.
The apparatus further comprises a particle or particle cluster sensing means used to monitor the shape of the particle or particle cluster and to generate a shape output signal for controlling the acoustic transducer means. The particle cluster sensing means monitors the average number density of a particle cluster.
At least one standing wave is generated by the acoustic transducer means and is used to create at least one pressure node for stabilizing the particle or particle cluster. The standing wave causes the particle or particle cluster to conform to a desired shape, and causes the particle cluster to conform to a desired average number density.
The radiation source comprises a radiation controller which determines radiation exposure time to induce a desired shape or density in the particle or particle cluster.
The controller comprises a timer, a shutter mechanism, and electronics capable of controlling the duration and intensity of exposure of the radiation source to the particle or particle cluster.
The object of the invention in part is to manipulate the droplet, particle or cloud shape to optimize or control the interaction of the radiation with the droplet, particle or cloud shape. Such shape control or molding can then be used in other processes where the treated droplet, particle or cloud of particles is deposited or disposed onto a surface of another object as a coating or layering.
The invention is also to be expressly understood as methods for using the above apparatus and for performing the functions of the apparatus disclosed above.
While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.